This invention relates to a blanket cylinder cleaner and more particularly to a device for automatically removing the debris which collects on the blanket cylinder of an offset lithographic printing press during the printing process.
The invention further relates to an automatic blanket cylinder cleaner which replaces the requirement of manual cleaning and which accomplishes cleaning consistently and expeditiously.
In order to maintain high quality printing, it is necessary to periodically clean the blanket cylinder of a printing press. Since the cleaning process necessarily results in press "down time," it is highly desirable that the amount of "down time" be minimized while at the same time obtaining effective cleaning and drying of the blanket cylinder.
It is known that excessive debris causes a deterioration in print quality. For this reason, it is necessary for the press operator to periodically interrupt the printing process in order to clean debris from the blanket. The procedure followed today in the vast majority of printing plants is to periodically interrupt the printing process and manually clean the blankets while jogging the press. This cleaning operation, which can take as long as ten minutes on a large sheet fed press, usually consists of three steps:
First, the blanket cylinder is wiped with a cloth dampened with water. Since the purpose of this setup is to remove water soluble paper debris, this step is omitted if there is no water soluble paper debris.
Second, the blanket cylinder is wiped clean of all ink using a cloth dampened with ink solvent. Alternatively, the cloth is dampened with water before dampening it with solvent.
Third, a fresh cloth is used to wipe the blanket dry. Although not essential to print quality, the drying step minimizes paper waste when the press begins operation. As a result, in most instances the operator takes the necessary time needed for the drying step.
The development of an automatic blanket cleaner involves several factors to be considered, including the nature of the solvent to be used.
In order to be effective an automatic blanket cleaner must be capable of washing the blanket cylinder with two types of solvent, one for water soluble debris and another for debris insoluble in water. Usually, the solvents are water and a hydrocarbon solvent. When washing debris which is water soluble, such as lint, clay coating and gum, plain water can be used. It has been found that a hydrocarbon solvent is required when the debris is ink based since this type of debris is not soluble in water.
It has also been found that the blanket cleaner must be designed so that it is capable of cleaning the blanket with a relatively mild solvent because a high strength solvent may damage the rubber covering on the blanket cylinder. Solvent power is determined by the KB (kauri-butanol) number. The lower the KB number, the weaker the solvent. A high KB solvent will be absorbed by the rubber blanket material and may cause swelling or damage. Although the maximum allowable KB number solvent which can be used will depend on the particular blanket being used, it is generally recommended that solvents having KB number of 30 or less be used. The automatic blanket cleaner system must also take into consideration that most hydrocarbon solvents used in blanket washings are not soluble in water. Accordingly, it has been found desirable that the system either have two separate solvent dispensers, or else have provision for generating and maintaining a water-solvent emulsion which can be used for cleaning.
A second problem involves the mechanical loosening of the debris on the blanket cylinder. Frequently, the debris which builds up on the blanket of an offset press adheres to the blanket tenaciously and cannot be easily removed. Thus, to perform under all conditions, an automatic blanket cleaning system must provide for a mechanical loosening or scrubbing action so as to dislodge debris which sticks to the blanket surface so that such debris can be removed.
A third significant factor is that the automatic blanket cylinder cleaning should significantly decrease cleanup time. On a sheet fed press, automatic washup time should not exceed two minutes and preferably should be no more than one minute. On a web press, the washup time should be even less in order to minimize the paper wasted during the washup.
A fourth factor to be considered is that the washup fluid must be confined to the blanket cylinder so as not to contaminate other press parts. It is particularly important that fluid not be allowed to enter the gap in the blanket cylinder where it can subsequently leak out and contaminate the blanket or plate during printing. Also, if solvent seeps under the rubber coating on the blanket cylinder it can cause blanket swelling and/or shorten blanket life. It is equally important to confine water when it is being used as a solvent since water can cause corrosion of press components.
A fifth factor is that the blanket cylinder must be completely dry before printing is resumed. Complete drying is particularly important when the washup liquid is solvent since solvent left on the blanket will contaminate the ink train and cause excessive paper waste on resumption of printing. Similarly, an excess of water left on the blanket following washup can disturb the ink water balance and result in increased paper waste.
Another factor to be considered for blanket cleaner devices is reduction in the consumption of solvent. One reason, of course, is reduction in cost by use of less solvent. However, in web press applications, lower solvent usage is also important in order to reduce the possibility of an explosion in the web press dryer. That is, most web presses used for commercial printing employ heatset inks which are dried by passing the printed web through an oven which drives off the ink solvents by evaporation. If an excessive amount of washup solvent is used, this solvent can cause an explosion when it is carried into the dryer by the web.
Another factor to be considered is reduction of the environmental impact of the cleaning operation. Where the waste material is in liquid form there can be serious problems in the disposal of the liquid wastes since there are many restrictions placed on disposal of such materials. Therefore, where possible, it is desirable that such waste material be in solid form.
Space conservation and compactness must also be considered in developing such equipment. There is very little space available on most printing presses for installation of automatic cleaning equipment. In addition, the installation of such equipment should not obstruct access to either the blanket or plate cylinders since both plates and blankets must be changed periodically.
It is important that the automatic blanket cleaner not generate any foreign particles (hickeys) which could contaminate the press. For example, if a cloth is used as part of the automatic system, it must not produce lint which could show up as hickeys in the subsequent printing operation.
Another significant feature required of automatic blanket cleaners is that they require a minimum of maintenance for reliable operation. This is particularly important on presses which are run around the clock since time spent on maintenance can reduce the time that a press is available for production. In addition, to be efficient the automatic blanket cylinder cleaner should complete its function in about two minutes, and preferably one minute, and should be even less on a web press.